Direct current (DC) power is needed for many types of telephone communication equipment, for control equipment used at electric utility substations, for computer data centers, power plants and other similar uses. The DC power may be supplied by a DC power source which may be supplied with AC power from an AC power source, such as the local power grid, or a generator and prime mover. Storage batteries may be utilized as a DC power source when a DC power source either cannot supply all the power required by the components or when the AC power supply or other external power source is not available, as during a power failure at the local electric utility or in the power distribution system. When the storage battery system is used as a backup power source, the batteries are recharged when the prime power or AC power is restored. In some cases the storage batteries are “floated” across the power bus so that they are continuously available.
The period of time where such battery backup is required may be reduced by providing local diesel-electric or turbine-powered electric generators. However, during the time where other backup power sources are unavailable or when switching between alternative prime power sources, standby storage batteries are needed.
A storage battery has an internal impedance, which includes resistive, inductive and capacitive components. When the battery is discharging, only DC is involved and the resistive component of the impedance is of interest as the discharge current produces a voltage drop across the internal resistance of the battery in accordance with Ohm's law. Over the life of the battery, the internal resistance will increase, at a rate determined by such factors as how many times the battery undergoes cycles of discharging and recharging, operating temperature, or the like. The internal resistance of any cell will eventually increase to a value where the voltage drop across the effective internal resistance during discharge is so great that the battery can no longer deliver power at its rated capacity. Other defects in the battery, or aging of the battery, may also result in degradation of the capacity of a battery to perform its function. Storage batteries may be subject to failure modes such as thermal runaway, degradation or failure of the outer case, internal short circuits, or the like. One or more of the installed storage batteries may need to be replaced during the lifetime of the backup power system. When series strings of storage batteries are used to increase the voltage being supplied or, in general, when batteries are connected in either series or parallel, the impedance of the overall string has an influence on the amount of energy that can be supplied. Other components of the physical assembly, including connecting links, terminal connections, supply cables and the like which can exhibit resistance, and have characteristics that may vary with time due to such factors as corrosion and changes in contact pressure, also contribute to the resultant battery system status. These ancillary components, including circuit breakers, sensors and the like, may also suffer from degradation and reduced performance or failure and require servicing.
Typical rechargeable storage batteries as used in a backup battery power supply system, for example, have a nominal terminal voltage of 12 VDC and a plurality of batteries may be connected in series so as to form a higher voltage power source. This is often done to reduce the current needed to provide a required power to the system being backed up, as the power is the product of the voltage and the current. But, this results in locations within a battery system where the voltage may be, for example, up to 480 VDC with respect to a ground or another location in the system. Such voltages are extremely dangerous to personnel and contact with such elevated potentials leads to serious injury and is often fatal. Consequently, safety regulations have been promulgated that may require safeguards against contact with elevated potentials, and which may require highly trained personnel, special procedures and formal maintenance permits. A maximum permitted voltage potential difference between accessible locations in a battery backup system is less than 50 VDC, without the special precautions or procedures. Other safety voltage limits may also be encountered.
Contact with the electrical circuit or components may be undesirable even in lower voltage applications where the high currents that the batteries are capable of supplying could lead to equipment damage, arcs, fire or other dangerous results.